1. Field of the Invention
The present invention relates to a method of forming an image comprised of a heat-diffusible coloring matter, and also to an image forming medium used in the method.
2. Related Background Art
Hitherto prevailingly employed as methods of making a record by heating are a direct thermal method, a thermal transfer recording method, a sublimation transfer method, and so forth.
The direct thermal method is a method in which a color-forming material having the property of forming a color by heat is heated in the desired pattern form with a thermal head or the like to form a character or image. In this method, the resulting image tends to cause color formation at the area in which no color has been formed, under the influence of heat or chemicals. This may disadvantageously result in a poorness in the stability or storage stability of images. It is difficult to produce a color image with this method.
On the other hand, the thermal transfer method is a method which employs an ink doner sheet formed of a plastic film made of polyester or the like coated with a heat-fusible ink containing a coloring material, and in which the doner sheet is heated in the desired pattern form with a thermal head or the like to bring the ink of the doner sheet into heat fusion and then the ink is transferred to an image receiving paper contiguously provided. An image is thus formed. In this method, plural colors can be overlapped to form a color image. A special unit or special doner sheet, however, is required for obtaining an image with density gradation.
The sublimation transfer method is a method which employs an ink doner sheet formed of a resin film coated with a heat-fusible ink containing a sublimating dye, and in which the doner sheet is heated in the desired pattern form with a thermal head or the like to bring only the dye at the heated area into sublimation, which is then transferred to an image receiving paper. An image is thus formed. Different from the above thermal transfer recording method, this sublimation transfer method enables control of the thermal head, control of heat-applying energy, control of heat-application time, thereby obtaining an image with density gradation.
FIG. 5 is a graph to exemplify the relationship between the temperature (Ttrans) of a heated ink doner sheet and the optical density (O.D.) of the coloring matter diffusion-transferred onto an image-receiving paper from the inside of the doner sheet, in a conventional sublimation transfer method. The coloring matter has a specific diffusion temperature (Tdiffuse) depending on the type of the coloring matter. In general, the smaller the molecular weight of the coloring matter is, the lower this Tdiffuse is; and the more polar groups the coloring matter has, the higher it is. Heating this coloring matter to a temperature not lower than the Tdiffuse (i.e. making the relationship of Ttrans.gtoreq.Tdiffuse) brings about diffusion of the coloring matter at the heated area and transfer thereof to an image receiving medium, and thus a image of coloring matter can be obtained on the image-receiving medium. In such a conventional sublimation transfer method, however, binders in the ink doner sheet have uniform physical properties and are by no means capable of suppressing the diffusibility of the coloring matter. For this reason, it has been not easy to control the sublimation of the coloring matter with good precision.
In addition, in the conventional sublimation transfer method, the thermal head repeats the on-off for heat generation at intervals of 1 msec and hence must be heated to a considerably high temperature before the desired sublimation and density can be achieved. Under heating at such a high temperature, however, it has sometimes occurred that not only the sublimating dye but also the binder component in the sheet are transferred to the image receiving medium to lower the image quality of the resulting image.